With the continuous progress of science and technology, fingerprint identification applications are used more and more widely, which has become one of essential functions in upscale smart phones. At present, the fingerprint identification is realized by a capacitive fingerprint detection chip in the smart phone. The fingerprint detection chip consists of a plurality of detection units and is packaged by plastic. Technical requirements for plastic packaging are very high because the fingerprint detection is a precise detection. The surface thickness of the plastic package is generally required to be in a range of 100 microns to 200 microns, and an error tolerance of the surface thickness of the plastic package for the same type products is required to be lower than 2 micron.
If the package thickness is too thin, the contrast of a detected image is high, which may be out of range; if the package thickness is too thick, the contrast of the detected image may be so low that an imaging effect is poor. In both of the above-mentioned situations, a poor efficiency of fingerprint detection may occur. Specifically, a fingerprint detection example is shown in FIG. 1. As shown in FIG. 1, reference numeral 0 represents a finger, reference numeral 100′ represents a fingerprint detection chip, reference numeral 113′ represents a detection unit, reference numeral 200′ represents a metal ring, and reference numeral 140′ represents a plastic package area. When a finger 0 is located on the fingerprint detection chip 100′, a coupling capacitance is generated between the finger 0 and the fingerprint detection chip 100′. The fingerprint detection chip 100′ detects the fingerprint by detecting the coupling capacitance corresponding to each detection unit 113′. The capacitance formed between the finger 0 and each detection unit 113′ of the fingerprint detection chip 100′ may be different if the package thickness is inconsistent. The thinner a package area is, the larger a corresponding capacitance is; the thicker the package area is, the smaller the corresponding capacitance is. A top view of a package structure of the fingerprint detection chip 100′ is shown in FIG. 2, in which the innermost area is the detection units 113′ for detecting fingerprint capacitance, an area surrounding the detection units 113′ is the fingerprint detection chip 100′, and an area surrounding the fingerprint detection chip 100′ is a metal ring 200′.
During operation of the fingerprint detection chip, a non-uniform package thickness may lead to different capacitance values corresponding to different detection units, such that difference between contrasts of images detected by the detection units is large, and all the detection units cannot operate in a desired state.